Continuous fiber reinforced composites are extensively used in both primary and secondary aircraft components for a variety of applications where light weight, higher strength and corrosion resistance are primary concerns. Composites are typically composed of fine carbon fibers that are oriented at certain directions and surrounded in a supportive polymer matrix. Since the plies of the composite material are arranged at a variety of angles, and depending upon the direction of major loading, the resultant structure is typically a stacked laminated structure, which is highly anisotropic and heterogeneous. A significant portion of the composite structure is fabricated as near net-shape, but is drilled in order to facilitate joining of components using mechanical fasteners. Drilling fastener holes in composite does not compare to the uniformity of aluminum or steel since individual carbon fibers fracture at irregular angles and form microscopic voids between the fastener and the hole. As the cutting tool wears down, there is an increase of surface chipping and an increase in the amount of uncut fibers or resin and delamination. The composite microstructure containing such defects is referred to as “machining-induced micro texture.”
Installation of fasteners into composite structures presents its own challenge as well, with the added risk of causing delamination between the plies, an issue not present in homogenous metallic structures. One source of delamination between the plies is too much interference of the composite structure with the fastener during installation. Another is delamination due to high fastener installation loads, in which the plies and mating epoxy become damaged during installation and thereby affect the mechanical integrity of the composite structure.
In addition to their machining and installation challenges, composite structures in aircraft are more susceptible to lightning damage compared to metallic structures. In part, this is due to the intrinsic electrical conductivity of composite structures, their multi-layer construction, and their anisotropic nature. For these reasons along with the hybrid nature of aircraft structures and relative conductivity of the various elements used in these structures, lightning protection of composite aircraft can become very complex.
Composite structures such as wings are usually constructed of a combination of metallic and composite structural elements which are joined together with metallic fasteners. While the metallic elements are good electrical conductors which efficiently dissipate high currents, composite elements are inefficient electrical conductors. The electrical field distribution and consequently the current density distribution associated with lighting strikes to hybrid structures like aircraft increases the importance and necessity for sufficient lightning management in aircraft.
Some estimates indicate that, on average, each commercial aircraft in service is struck by lightning at least once per year. It is also well established that metallic fasteners are often both the preferred point of lightning attachment and a primary pathway for lightning current. Aircraft flying in and around thunderstorms are often subjected to direct lightning attachment as well as indirect lightning effects which occur from the attachment of lightning current elsewhere on the aircraft and the subsequent flow of lightning current across the aero-structure interface. This often results in multiple points of attachment, and lightning currents which indirectly interact with fasteners other than the primary and secondary points of attachment.
Fasteners often act as pathways for the conduction of the lightning currents from the skin of the aircraft to supporting structures such as spars or ribs. As such, both modes of attachment can result in damage to the fastener and the area of the structure surrounding the fastener as the current density is significantly increased locally. The struck fastener often interacts with the surrounding structure in a manner that results in the ejection of hot particles from the fastener/structure interface. This condition is commonly referred to as Hot Particle Ejection or “HPE.” HPE can be caused by both direct and indirect lightning effects, but the risks associated with direct effects are generally perceived to be higher. HPE can occur in composite, hybrid or aluminum structures.
The portion of the aircraft which is of greatest concern is the area near the fuel tank where both the risk of HPE and its consequence are perceived to be highest. Since commercial aircraft contain relatively large amounts of fuel and also include very sensitive electronic equipment, they are required to comply with a specific set of requirements related to lightning strike protection in order to be certified for operation. As such, HPE could create a detrimental ignition source capable of compromising the integrity of the fuel system. Thus, there is a need for resilient and cost-effective countermeasures to HPE.